Mobile phones and other hand-held electronic devices, for example, game controllers require a vibration source that is detectable by the sense of touch of the user. These vibrations signal the user of events without creating significant audible sound. For example, such events might include an incoming phone call, incoming text message, the activation of a button on a flat screen display, turbulence of a virtual aeroplane in a computer video game, and many other functions. The vibration source must be sufficiently strong to be felt by a person holding the device. These sources are most generally referred to as vibration motors or haptic actuators.
One common type of vibration motor is an eccentric rotating mass (ERM) motor with a rotating shaft and an unbalanced mass attached to the shaft that generates oscillating centripetal force perpendicular to the axis of rotation. More than one billion ERM motors are manufactured each year; the typical rotation speed is 100 to 300 Hz, and the typical centripetal force is 0.1 to 1 N.
Exemplary electromagnetic ERM motors include the Model NRS-2574i produced by SANYO SEIMITSU CO., LTD. and the Model DMJBRK30X produced by SAMSUNG ELECTRO-MECHANICS CO., LTD. Some versions are a tubular type ERM motors, and some are disk type ERM motors. For example, some of the smallest tubular ERM motors are about 4 mm in diameter and 6 mm in length, with a shaft and unbalanced Tungsten mass extending about 4 mm from one end of the motor. The smallest disk type ERM motors are 10 mm in diameter and 3 mm thick, with the Tungsten mass rotating inside the motor housing and the rotation axis parallel to the centerline of the 10 mm diameter. For both types of motors, a torque is generated to rotate the shaft using conventional direct current (DC) motor designs that include copper coils, iron cores, permanent magnets, and coil switching using brushes and armature. Tungsten is used for the mass because its density is more than twice the density of steel. For a tubular motor, a typical Tungsten mass is 0.4 grams with a center of gravity offset 1 mm from the centerline of shaft rotation. For this example, when the mass rotates at say 200 Hz (1,256 Rad/sec), the generated centripetal force Fc=Mass×(Angular Velocity)2×(Radius of Offset)=0.0004 kg×(1256 Rad/sec)2×0.001 M=0.63 N. This dynamic force is sufficient to accelerate the entire mobile phone handset and create vibrations that are perceived by the user.
Another type of vibration motor is a Linear Resonant Actuator (LRA) in which a Tungsten mass is suspended by spring-guide system that allows movement along a substantially linear path, and the spring force acts to keep the mass in the center of the path. An electromagnetic coil and magnet generate Lorentz forces that move the mass back and forth along the path at a frequency equal to the resonant frequency determined by the mass and stiffness of the spring. By operating at resonance, this actuator generates a large vibration amplitude using a relatively small power input to the electromagnetic coil. An example of an LRA motor is the Model DMJBRN1036AA device from SAMSUNG ELECTRO-MECHANICS CO., LTD.
A limitation of ERM and LRA electromagnetic vibration motors is they produce magnetic fields and are constructed of ferromagnetic and conductive materials. The magnetic interference produced by these motors interferes with the operation of other devices in mobile phones (e.g., a compass). This is especially problematic as mobile phone handsets add additional devices and also continue to become smaller and more integrated. Electromagnetic motors are also made from conductive materials that are not transparent to radio frequencies (RF) and cannot be located near a radio antenna of a wireless communication device.
A further limitation of the electromagnetic ERM and LRA haptic actuators is the need for a large percentage of the motor structure to be stationary (e.g., either the windings or the magnet must be stationary). The stationary mass does not contribute to the acceleration force generated by the haptic actuator and increases the total size of the device.
Ceramic motors, such as piezoelectric ultrasonic motors, do not generate magnetic fields, can be constructed from non-ferromagnetic materials, and can also be made almost entirely from non-conductive materials that are substantially RF transparent. A non-magnetic, RF transparent motor has many advantages for integration in highly miniaturized mobile phones.